A method is provided for measuring the aerodynamic particle size distribution of airborne particles by drawing the particles through a device which continuously or intermittently changes the aerodynamic cut of the particles entering an optical sensor. Optical sensing is performed by aerosol photometry or optical single particle size spectrometry. If aerosol photometry is used, the aerosol photometer becomes an aerodynamic particle size spectrometer by relating the aerosol photometer's output to the aerodynamic particle size fractions passing through the aerodynamic cut device. If optical single particle size spectrometry is used, the optical single particle counter becomes an aerosol particle size spectrometer by calibrating the optical sizes of the optical single particle counter relative to the aerodynamic particle sizes determined by the aerodynamic cut device.